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Fig. 6.35 A-D. Frontal aspergillosis complicating a paranasal infection. A An axical CT scan after enhancement. There is a mass in the right frontal region obliterating the anterior horn of the right lateral ventricle and compressing the left anterior horn. B The same patient. The coronal scan (bone window) shows the soft tissue mass occupying the right maxillary and ethmoidal sinuses: it has destroyed the medial wall and expanded into the orbit, displacing the external ocular muscles. Postoperatively, MRI (T2-weighted) shows a residual mass in the right maxillary antrum (C) and in the right ethmoidal sinus and right orbit (D). The aspergilloma showed intermediate-low signals in both T2- and T1- weighted sequences. (In D there is a rim of fluid over the right temporal region.) The patient was a 39-year-old female from Pakistan with a short clinical history of headache, nausea, vomiting, and deteriorating vision in the right eye. The right antrum had been drained 10 years previously. At the time of A and B there was right-sided proptosis, right optic atrophy, and left papilledema. Histology showed numerous fungal hyphae of Aspergillus flavus. (Courtesy of Dr. A Coulthard et al. and Clin Radiol, 1991)

 

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Imaging Diagnosis

The chest and paranasal sinuses are the niost frequent sites of Aspergillus spp. infection, from which dissemination originates.

Pulmonary Aspergillosis

In pulmonary aspergillosis the imaging findings depend on the reaction of the patient to the fungus. There are four main categories:

1. Hypersensitive (allergic) aspergillosis
2. Noninvasive aspergillosis
3. Semi-invasive aspergillosis
4. Invasive aspergillosis

1. Hypersensitive aspergillosis is, worldwide, probably the most common reaction to the fungus, which in these patients is noninvasive. The host becomes sensitized and, in the lungs, there is a severe inflammatory reaction which leads to bronchitis and bronchiectasis. Many patients are asthmatic, and in many of them there are no clinical symptoms, but there is an abnormal chest radiograph. The imaging diagnosis will be bronchiectasis and the dilated bronchi often appear as linear or Y-shaped "fingers" (cigar or banana densities) because of the thick mucoid material (mucous plugs) within them (Fig. 6.33). This is of relatively high density because of the compacted fungal elements. CT (and bronchography) shows that the bronchiectasis is most marked in the larger bronchi, the more peripheral bronchi being normal. Because of plugging, there will be radiographic evidence of lobar or segmental atelectasis and of obstructive emphysema. The findings are very variable, sometimes changing every day in the same patient. Fibrosis is often the end result.

Aspergillus is one of the less common causes of a hypersensitivity reaction in the lungs (other causes include farmer's lungs, pigeon breeder's disease, byssinosis). In the acute phase there may be pulmonary edema but more commonly there is a fine nodular interstitial pattern throughout both lungs, over which there may be flame or triangular densities, often rapidly appearing. When there are small granulomas surrounding the bronchi (bronchocentric granulomatosis), hyphae of Aspergillus spp. can be identified in many of them histologically.

2. Noninvasive aspergillosis: the saprophytic form of aspergillosis always occurs in previously damaged lungs, usually in a cavity. Imaging shows a mobile intracavitary, usually indistinct "ball-shaped mass" (Fig. 6.34). On plain radiographs, erect and decubitus frontal projections will usually show movement of the aspergilloma. In some patients either standard tomography or CT will be necessary. Similar round masses may be due to other fungi, a thrombus without infection, a tumor, or Wegener's granulomatosis. A further difficulty is that fungal infection can be superimposed on another underlying cause. Because the cavity may react to the fungus ball, with vascular granulation tissue and in some patients a pleural reaction, it is sometimes difficult to be sure that the aspergilloma is still noninvasive. In patients with advanced preexisting tuberculosis or other lung disease, the differentiation between invasive and noninvasive aspergillosis may depend more on the clinical and laboratory tests than on imaging.

3. Semi-invasive aspergillosis has only been recognized in the last two decades, and probably occurs only in patients who have a depressed immune system. In this form, it is the Aspergillus which creates a chronic cavity and goes on to form an aspergilloma. This is best documented by serial imaging, which usually shows alveolar consolidation which goes on to cavitate as a result. of peripheral infarction and necrosis. There is often an extensive pleural reaction. Later a thick-walled, irregular cavity will be seen, which in some patients then becomes thin walled: the lining may be irregular without a fungus ball, or become smoother and eventually contain a typical aspergilloma. Again, it is difficult to decide between the noninvasive and semi-invasive pattern of infection. Clinical and laboratory correlation are important and serial images are equally necessary.

4. Invasive aspergillosis: only occurs in immunocompromised patients and results in a wide variety of imaging patterns. The initial diagnosis may be pneumonia, sometimes in more than one lobe, but in many patients there are multiple, ill-defined pulmonary nodules. These are probably the result of pulmonary emboli containing fungi. When there is a large fungal embolus (which is not immediately fatal), consolidation will be found on imaging, which breaks down and cavitates into an abscess. It may be difficult in some patients to decide whether the "mass" is due to necrosis or whether there is now a cavity containing a fungus ball. When the patient is severely ill, the lung may become diffusely involved with multiple irregular (necrotic) spreading nodules: the pleura, pericardium, and even the esophagus and trachea may be involved.

Invasive respiratory tract aspergillosis is a well-recognized complication in severely immunocompromised patients. In the early stages, the chest radiograph may remain normal or show only minimal patchy densities. The airways develop ulcers and inflammatory plaques which do not extend into the lungs. The subsequent pulmonary changes are probably the result of the thick mucus which develops and causes airway obstruction.

In almost all pulmonary infections, the initial problem is to decide whether the Aspergillus is a contaminant or a cause. CT scanning of early pulmonary aspergillosis, when there are peripheral nodules, shows a halo of attenuation around these focal areas of bronchopneumonia. This is probably caused by growth of the Aspergillus in a ring pattern. The description "target lesions" has been used, and is a consequence of a necrotic center, surrounded by a hyperemic perimeter containing invading hyphae. Part of the blurred ring seen radiographically, and better recognized on CT scanning, may be the result of edema. Nodules and the cavitation within them may be seen on CT scanning but may not be visible on conventional radiographs. Indium-labeled white blood cell scintigraphy may also show infection which cannot be seen on a chest radiograph.

Miliary microabscesses have been reported as a rare form of disseminated aspergillosis: there is no cavitation or infarction. These may be recognizable on high-resolution CT scanning.

CT scanning is the best way to record developing cavitation as the infection progresses. Air within a cavitating mass may indicate shrinkage of the central necrotic tissue and may be an early sign of healing. However, there can be other explanations for this process and the clinical condition of the patient may be a more reliable guide to progress.

In brief, radiographs will clearly show the overall pattern; CT will show the complications more clearly, but the clinical state of the patient and the laboratory results must make the definitive diagnosis.

 

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Copyright: Palmer and Reeder